This invention relates generally to a method/system of white-light density pseudocolor encoding, and, more particularly, to a technique of generating density pseudocolor encoding with three primary colors using a white-light optical processor.
Most of the optical images obtained in various scientific applications are usually in the form of gray-level density images. For example, scanning electron microscopic images, multispectral aerial photographic images, x-ray transparencies, etc. are all gray-level images. It is well recognized, however, that humans can perceive in color better than they can perceive gray-level variations. In other words, a color coded image can provide the viewer with a greater ability for visual discrimination.
In current practice, most of the pseudocolorings are performed by use of a digital computer technique such as the type disclosed in an article by Andrews, H. C. et al, "Image processing by digital computer," IEEE Spectrum, July 1972, pp 20-32. Such a computer technique is a logical choice if the images are initially digitized. However, for continuous tone images, optical color encoding techniques such as the type described by the present inventor, Yu, F.T.S., Optical Information Processing, Wiley-Interscience Publishing Company, New York, 1982 would be more advantageous for at least the following three major reasons: (1) the technique and principle can preserve the spatial frequency resolution of the image to be color coded; (2) the optical system is generally easy and economical to operate; (3) the cost of an optical pseudocolor encoder is generally less expensive when compared with the digital counterpart.
Density pseudocolor encoding by half-tone screen implementation with a coherent optical processor was first reported by Liu, H. K. et al, "A New Coherent Optical Pseudo-Color Encoder," Nouv. Rev. Opt., t. 7, no. 5, 1976, pp. 285-289 and later with a white-like processor by Tai, A. et al, "White-light pseudocolor density encoder," Opt. Lett., Vol. 3, Nov. 1978, pp. 190-192. Although good results have been subsequently reported there is a spatial resolution loss with the half-tone technique and a number of discrete lines due to sampling are generally present in the color-coded image.
Recently a technique which offers the advantages over the half-tone technique has been developed. This is a technique of density pseudocoloring through contrast reversal. An example of this technique can be found in an article by Santamaria, J. et al, "Optical Pseudocoloring Through Contrast Reversal Filtering," J. Opt., Vol. 10, no. 4, 1979, pp 151-155. Although this density pseudocoloring technique offers several advantages over the half-tone technique, the optical system required therewith is more elaborate and requires both incoherent and coherent sources. Since a coherent source is utilized in such a system, coherent artifact noise is unavoidable.
More recently, the present inventor developed a density white-light pseudocolor encoding technique as set forth in an article by Chao, T. H. et al, "White-light pseudocolor density encoding through contrast reversal," Opt. Lett., Vol. 5, No. 6, June 1980, pp. 230-232. This recent technique offers the advantages of coherent noise reduction, no apparent resolution loss, versatility and simplicity of system operation, and low cost pseudocoloring. Although excellent results have been reported by this technique, the pseudocolor encoding is primarily obtained by means of two primarily colors. More specifically, in this technique spatial encodings are made through positive and negative photographic image transparencies, and the pseudocoloring is obtained by color filtering of the smeared Fourier spectra.
Unfortunately, limiting the above-mentioned technique of density white-light pseudocoloring to only two primary colors constitutes a drawback in visual discrimination between images being viewed. It would therefore be highly desirable to provide a pseudocolor encoding technique capable of providing an even more discriminating color coded image.